In a cellular network mode, data traffic usually goes through centralized controller such as a base station (BS) even if the communicating devices are close to each other. One benefit of the centralized operation is easy resource control and interference control. One potential drawback is less efficient resource utilization. A double amount of resources may be needed for a user equipment (UE) in a cellular mode. For example, a cellular UE may need to have a first radio resource allocated between the cellular UE and the base station and a second radio resource allocated between the base station and the other cellular UE. Examples of the centralized controller may include evolved Node B (eNB) of long-term evolution (LTE).
In comparison, a UE in a direct, device-to-device (D2D) mode may only need one radio resource between the UE and a pairing UE, when two users are sufficiently close to each other. To help improve system throughput, a radio network may operate in a D2D mode. A UE may operate in the cellular mode if the other party is located far way and may operate in the D2D mode if the pairing UE is sufficiently close by. Mixed cellular and D2D modes are being explored in new generations of wireless technologies such as 3rd generation partnership project (3GPP) long-term evolution (LTE), LTE-advanced (LTE-A) and worldwide interoperability for microwave access (WiMax) networks. One example of D2D mode network is an ad-hoc network where one D2D UE may set up a direct connection with the pairing UE via a handshake and competition procedure. Examples of cellular networks include widely deployed wireless networks such as universal mobile telecommunications system (UMTS) network, code division multiple access (CDMA) network, WiMax network and 3GPP LTE networks.
It may become common to switch between the cellular communication mode and the D2D communication mode to accommodate a high degree of mobility, to support application and services such as location-based social networking, and to achieve more efficient resource utilization. The communication mode switch may involve the packet data convergence protocol (PDCP) that may perform packet header compression and decompression, transfer of user data and maintenance of protocol data unit (PDU) sequence numbers for radio bearers.